If memory serves the Crystal Skull actually makes an appearance in all Max McCoy's novels, even though it's not the "MacGuffin" of any. Indy has a disturbing vision with it and leaves it there. I think it involved CGI prairie dogs...

Oh, and you missed out the most recent novel: Indiana Jones and the Army of the Dead, featuring voodoo, zombies, and (rather unnecessarily) Mac. There was also another Rob McGregor novel, Indiana Jones and the Staff of Kings, that tied in to the videogame, but sadly was cancelled even though it's complete. I wish you guys had covered the main videogames. Fate of Atlantis in particular should've been the fourth film.